Air is indispensable for the living of human beings. Fresh and clean air is essential for the health of people. The remarkable advancement of science and technology has brought the prosperous development of economics. However, the exhaust emission of automobiles, factories and other combustion sources has become the main source of air pollution and greatly degraded the quality of air.
Take automobiles for an example. The vehicle exhaust emission standard is growing more and more stringent. However, the air pollution caused by vehicles is still getting more and more serious because the number of vehicles increases day by day. Generally, different types of fuels are burned in a cylinder to release thermal energy and generate dynamic power for engine operation of an automobile. However, the combustion of fuel also generates exhaust gases including harmful pollutants, such as nitrogen oxides (NOX), carbon monoxide (CO), hydrocarbons (HCs), particulate matter (PM) and the like. These pollutants not only form photochemical smog, but also destroy the ozone layer, aggravate the greenhouse effect, cause acid rain, damage ecological environment, and harm health of human beings.
Incomplete combustion generates carbon monoxide, which combines with hemoglobin to form carboxyhemoglobin (COHb). The hemoglobin's combination affinity for carbon monoxide is 300 times greater than that for oxygen. Thus, too high a concentration of carbon monoxide in air will affect the oxygen delivery ability of hemoglobin. A nitrogen oxide comes from the chemical combination of nitrogen and oxygen and is usually exhausted in form of nitric oxide (NO) or nitrogen dioxide (NO2). After illuminated by ultraviolet (UV), nitrogen oxides react with hydrocarbons to form toxic photochemical smog which has special smell, irritates people's eyes, injures plants, and reduces visibility in atmosphere. Also nitrogen oxides react with moisture in the air to form nitric acid and nitrous acid which are elements of acid rain. A low concentration of hydrocarbon will irritate the respiratory system. A higher concentration of hydrocarbon will affect the function of the central nervous system.
Therefore, many regions, including European Union, USA, Japan and the Republic of China, have established stringent exhaust emission standards, such as US Bin5 and Euro6, which regulate the exhaust emission standards of nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HCs) and particulate matter (PM) to control and decrease the emission of polluting gases and encourage the manufacturers to produce, develop or introduce the latest products adopting pollution prevention technologies.
In the conventional emission control technology for lean burn exhaust, no single device or converter can simultaneously undertake the conversion of nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HCs) and particulate matter (PM). Catalytic converters of automobile emission systems for the lean burn exhaust can catalyze the oxidation of carbon monoxide and hydrocarbons. Catalytic particulate filters can trap and oxidize the particulate matter. Another auxiliary device or system is needed to convert nitrogen oxides. For example, in addition to an oxidizing catalytic converter for oxidation of carbon monoxide and hydrocarbons, most of the tailpipes of current diesel vehicles are equipped with an EGR (Exhaust Gas Recirculation) system to control emission of nitrogen oxides. The latest diesel vehicles may be equipped with an SCR (Selective Catalytic Reduction) system to reduce nitrogen oxides.
The SCR system adopts ammonia (NH3) or aqueous solution of urea (CO(NH2)2) as the reactant. The aqueous solution of urea is injected into the tailpipe via a nozzle and reacted with water to form ammonia. The ammonia reacts with nitrogen oxides to generate nitrogen (N2) and water (H2O). However, ammonia is toxic and hard to store and may leak. The incomplete reaction of ammonia causes secondary pollution. Further, the SCR system is bulky and needs to be equipped with precision sensors to provide auxiliary control.
An U.S. Pat. No. 5,401,372 discloses an “Electrochemical Catalytic Reduction Cell for the Reduction of NOx in an O2-Containing Exhaust Emission”, which is a device dedicated to remove nitrogen oxides, and which utilizes electrochemical catalytic reduction reaction and adopts vanadium pentaoxide (V2O5) as the catalyst to convert nitrogen oxides into nitrogen. The device has to add a power supply to operate the electrochemical cell thereof. Thus the device of this prior art not only consumes energy resources, but also cannot remove harmful exhaust gases simultaneously.
An U.S. application Ser. No. 12/774,180 proposed by the same Inventor and entitled “Electrochemical-catalytic Converter for Exhaust Emission Control with Power Generation” discloses a converter to convert the exhaust gas into the electric energy, not only the harmful components in the exhaust gas can be converted into harmless components, but also power generation can be achieved. However, the converter in this prior art is additionally supplied by the external fuel to operate a solid oxide fuel cell (SOFC) in the converter, the fuel is consumed and a fuel processing system has to be installed on the vehicle. Thus the fixed and operating costs are increased, and the lifespan of the device is shortened due to accumulation of carbon caused by the fuel.